Granite wash reservoirs in Oklahoma, Texas, Colorado and New Mexico have proven to be effective commercial producers of hydrocarbons. The granite wash, an arkose, consists of many localized potential reservoirs which vary greatly in thickness and lithology. An arkose is defined as a sandstone containing 25% or more of feldspar usually derived from the disintegration of acid igneous rocks of granitoid texture. Commercially productive zones consisting of quartz, feldspar, calcite, mica and clay mixtures exhibit typical reservoir characteristics such as adequate permeability, low clay content and sufficient porosity.
In order to correctly evaluate the potential of a rock formation, it is necessary to obtain a lithologic profile. Many rock formations, such as granite wash, are complex. The composition at any interval is typically a function of many variables. Logging tools, such as neutron logs, density logs, and resistivity logs, and other indirect measurements have been used to measure various characteristics of rock formations in attempts to determine crude lithology.
Lithologic changes in rock formations affect the responses of standard logging tools in different ways. It has previously been considered impossible to obtain a precise lithology from logging tools alone, except for two member systems having a known fluid content. In order to evaluate more complex rock formations, it has been necessary to make assumptions about the logging tool responses which have been extrapolated from experience in simple systems. Unfortunately, as with any assumption, inaccurate evaluations occur. Consequently, productive zones are often overlooked, and nonproductive zones are confused with highly productive ones.
In order to determine lithology, the specific amounts of certain elements in the rock formation must be known. This information can be obtained by physically removing a sample of rock from a formation and returning it to the surface for analysis. The most common techniques available for doing this are cuttings, sidewall coring, and conventional coring.
Rock cuttings are ten-foot samples taken of the rock formation which are circulated to the surface by the drilling fluid (mud) system where they can be examined in the field or sent to a laboratory. This approach is limited in that serious problems such as lag time, contamination and sloughing frequently occur. In addition, due to the heterogenous nature of complex reservoirs, mere ten foot samples may not be representative of the interval in question.
Sidewall coring is also available. With this method, operators select specific intervals to be sampled, which are taken from the side of the borehole and returned to the surface for evaluation. High compressive strength formations make sidewall cores difficult to recover and their cost is moderate to high considering the number of intervals which need to be adequately tested.
Conventional coring yields the most complete information on cores brought to the surface. The core will indicate lithology, porosity, permeability, and so forth. Unfortunately, coring is the most expensive method available. Hence coring the entire well is certainly impractical. As a result, geologists and engineers must designate the intervals to be tested while drilling. This has been difficult as there is little information available about any given rock formation. If the core is taken too early, it will need to be repeated further down the borehole, increasing the cost. On the other hand, a late core will miss the productive zone. All too frequently, coring results in recovery of shale sections or other unwanted zones.
In each of the above methods, samples must be physically removed from the formation and returned to the surface for evaluation. It would be desirable to develop a method for accurate lithologic determination using in situ measurement.
A technique of spectral analysis of rock formations is relatively new and has not yet achieved widespread field application. Spectral analysis involves the interpretation of spectra obtained from rock formations using logging tools including a passive gamma ray detector and a neutron induced gamma ray log. In the latter tool, a neutron source is placed alongside the formation and periodically emits bursts of high energy neutrons to excite the atoms in the formation. A detector records the number of counts of returning gamma rays and segregates them according to their energies.
At the present time the information on a neutron induced gamma ray spectrum is largely background noise, for the most part due to compton scattering. Spectral analysis is currently being used to provide ratios of the elements carbon and oxygen which are interpreted to indicate the presence of hydrocarbons or water. Due to the high background noise level in a neutron induced gamma ray spectrum, it has not previously been possible to determine specific amounts of individual elements from spectral interpretation.
When a high-energy neutron bombards a rock formation, the neutron can collide "inelastically" with an atomic nucleus and transfer energy to it. The nucleus assumes an excited state, decays and emits a quantum of gamma radiation which is recorded by the logging tool in an "inelastic" spectrum.
The neutron can interact further with the atoms in the formation by other mechanisms. The neutron can lose energy by random thermal scattering with various atoms (so-called compton scattering). Eventually most neutrons will be captured by formation nuclei, leaving each such nucleus in an excited state. The excited nucleus, in most cases, almost instantaneously decays and emits a quantum of "capture" gamma radiation at an energy which is characteristic for each type of nucleus. Such "capture" gamma radiation is recorded by the logging tool in a "capture" spectrum.
The signal on either a capture or inelastic spectrum records the magnitude of gamma radiation as a function of energy. Due to the quantum nature of matter, each element emits gamma radiation at characteristic energies. The energies involved in inelastic or capture interactions are sufficiently large relative to the neutron's initial energy that one gamma ray is emitted with each event.
There is a direct correspondence between the number of gamma rays emitted and the number of atoms interacting with the neutrons in capture or inelastic scatter events. If the number of counts of gamma rays at particular energies could be ascertained, it would be possible to quantitatively determine the amount of each element present in the formation.
However, at the present time it has not been possible to determine count numbers from neutron logging data because compton scattering dominates the entire ranges of both the capture and the inelastic spectra and can exceed 90% of the recorded information.
When compton scattering occurs, the neutron dislodges an electron from the interacting atom. The freed electron initiates a chain reaction and dislodges many other electrons in the process of reaching thermal equilibrium with the formation. The newly dislodged particles in turn dislodge other particles. These interactions produce a cascade of gamma rays which is detected by the logging tool.
The number of gamma ray counts attributable to compton scattering does not correspond to the number of atoms in the formation because of the cascading effect. Unfortunately, compton scatter permeates the entire spectrum recorded by the logging tool. This has previously made it impossible to correlate count numbers with the amount of any given element in the formation.
The compton effect varies from place to place over the spectrum, depending on the particulars of a large number of interactions, which are randomly produced. Compton scattering is also variable depending on borehole conditions, tool positioning and design, the presence of drilling muds, and other factors. Hence, one cannot merely measure relative peak heights from the spectrum to determine count numbers. It would be desirable to develop techniques for enhancing neutron logging data to reduce compton scattering noise.
The limited resolution of detectors in neutron logging devices has also contributed to the background noise problem. There are elements in the formation whose gamma rays of capture or inelastic scatter possess energies close to energies of gamma rays from elements of interest. These "spurious" gamma rays are sometimes counted by the detector at the energies of interest and contribute to broadening of the signal peaks. Conversely, a portion of the desired "count" is sometimes assigned an incorrect energy by the detector. It would be desirable to develop techniques for enhancing neutron logging data to minimize this problem.
The "Carbon/Oxygen" neutron logging tool was developed to determine whether hydrocarbons or water are present in a formation by ascertaining whether capture and inelastic spectra indicate the presence of carbon and oxygen. No attempt has previously been made to determine the percentage amounts of these elements due to the compton scattering noise problem. The Carbon/Oxygen spectra have been previously used to supply a crude ratio of carbon peak to oxygen peak to indicate the presence or absence of oilbearing zones (hydrocarbons). By taking a ratio of peaks, the compton scatter is proportionately reduced. However, the usable information is in the form of a ratio of peak height of one element to peak height of a different element. Such ratios alone cannot determine the count numbers or percentage amounts of any elements independently of each other.
It would be desirable to develop a technique of in situ measurement of rock formations which indicates the correct amounts of certain elements in the rock formation so that an accurate lithology can be determined. It would also be desirable if such a technique were applicable to complex reservoirs where lithology is a function of many variables.